Tire Offset Calculator
A new set of wheels can transform a car, or rub, poke and scrape if the fitment is wrong. Width alone never tells the whole story, because offset decides where that width sits. The Tire Offset Calculator compares your current wheel with a new one using width and offset. It works out backspacing and outspacing for both wheels, then shows the poke change in millimetres and inches. A simple verdict band helps you judge the result at a glance. Drivers, tyre shops and anyone planning a wheel upgrade can use it before spending a cent.
Wheel Offset Explained Simply
Offset is the distance between a wheel’s centre line and its mounting face. It is measured in millimetres and stamped on the wheel as an ET figure. A positive offset tucks the mounting face toward the street side of the wheel. That pulls the whole wheel inward under the arch. A negative offset pushes the wheel outward for a deep, aggressive stance. Zero offset places the mounting face right on the centre line.
Positive, Negative and Zero Offset
Most modern road cars use positive offsets, often between plus 30 and plus 50. High positive numbers keep wheels tucked safely inside the arches. Negative offsets appear on lifted trucks and show cars chasing a wide look. Changing offset by even 10 millimetres visibly moves the wheel. It also changes clearance to suspension parts on the inside. Both directions carry limits that your car decides, not fashion.
Why Width Changes the Equation
A wider wheel grows on both sides of its centre line when offset stays fixed. Half of the extra width moves inward and half moves outward. That is why a wider wheel at the same offset can still rub inside. Comparing two wheels therefore needs width and offset together. Looking at either number alone leads to the classic fitment mistakes. This calculator always treats the pair as one combined measurement.
Backspacing and Outspacing
Backspacing and outspacing describe the same wheel from opposite sides. Together they map exactly where the rim edges will sit. Fitters have used these two figures for decades because they translate offset into plain distance. The sections below define both terms.
Backspacing: The Inner Measurement
Backspacing measures from the mounting face to the inner edge of the rim. To find it, take the wheel width plus one inch, halve it, then add the offset in inches. The extra inch accounts for the rim flanges beyond the bead width. A bigger backspacing number means the wheel reaches further inward. That inward reach is what can meet struts, arms and inner arches. Note the figure whenever you compare wheels for tight engine bays.
Outspacing: The Outer Measurement
Outspacing is whatever width remains on the street side of the mounting face. Subtract backspacing from the total width, meaning the width plus one inch. A bigger outspacing figure pushes the rim edge toward the arch lip. That outer position is what people call poke when it grows. It shapes the stance you see in the mirror. It also decides whether the tyre shoulder clears the fender under load.
Poke Change and Inner Clearance
The poke change is simply new outspacing minus old outspacing. A positive figure means the new rim edge sits further out. A negative figure means it tucks further in than before. Inner clearance change runs the opposite way, from old backspacing minus new backspacing. A negative inner figure warns that the new wheel reaches deeper inward. Reading both numbers together gives the full fitment picture in seconds.
How to Use the Calculator
You need four numbers, all printed on your current and future wheels.
- Find the current wheel width in inches, from the wheel’s stamped markings.
- Find the current offset in millimetres, shown as the ET figure.
- Enter the new wheel width you are considering, in inches.
- Enter the new wheel offset in millimetres from its listing.
- Press Calculate and compare backspacing, outspacing and poke change.
- Read the verdict band before you commit to buying the wheels.
Worked Example 1: A Subtle Width Upgrade
Inputs
A hatchback runs an 8 inch wheel with a plus 35 offset. The owner wants a 9 inch wheel with a plus 45 offset. Both figures come straight from the wheel listings. The change looks mild on paper.
Calculation
For the current wheel, total width is 9 inches, and half of that is 4.5 inches. Adding 35 millimetres, or about 1.38 inches, gives a backspacing of 5.88 inches. Outspacing is 9 minus 5.88, which is 3.12 inches. For the new wheel, total width is 10 inches, half is 5 inches, and 45 millimetres adds about 1.77 inches. Its backspacing is 6.77 inches and outspacing is 3.23 inches.
Result
The poke change is only plus 0.11 inches, about 2.70 millimetres outward. The verdict reads near stock fitment. However, the inner clearance change is minus 0.89 inches. That means the wider wheel reaches notably deeper inside the arch. The owner should check strut clearance before buying, despite the calm outer result.
Worked Example 2: A Noticeable Stance Change
Inputs
A coupe runs a 7.5 inch wheel with a plus 40 offset. The planned wheel is 8.5 inches wide with a plus 30 offset. Lower offset and extra width both push outward here. This example shows how the effects combine.
Calculation
The current wheel has a total width of 8.5 inches, and half is 4.25 inches. Adding 40 millimetres, about 1.57 inches, gives backspacing of 5.82 inches. Its outspacing is 8.5 minus 5.82, which is 2.68 inches. The new wheel totals 9.5 inches, half is 4.75 inches, and 30 millimetres adds about 1.18 inches. Its backspacing is 5.93 inches and outspacing is 3.57 inches.
Result
Here the poke change is plus 0.89 inches, a full 22.70 millimetres outward. The verdict reads noticeable poke increase. Inner clearance barely moves, at minus 0.11 inches. Attention therefore shifts to the outer arch and tyre shoulder. A test fit or careful arch measurement is wise before ordering tyres.
Offset, Tyres and Suspension Together
A wheel never works alone on the car. Tyre width, sidewall height and suspension travel all share the same tight space. A rim that clears at rest may touch at full lock or full bump. Lowered cars shrink every margin the factory left behind. This is why experienced fitters think in complete packages, not single parts.
Tyre Choice Can Steal or Save Room
Two tyres with the same marked size can measure differently in reality. Sporty tyres often run wide, with squared shoulders that reach further out. Touring tyres tend to run narrower and rounder at the edges. A stretched tyre on a wide rim pulls the shoulders inward noticeably. A chunky all terrain tyre does the opposite and eats clearance. When margins look tight, research the actual measured width of your chosen tyre.
Suspension Travel and Steering Lock
Clearance must survive the worst moment, not the average one. Full steering lock swings the tyre shoulder through a wide arc. A deep pothole compresses the suspension and lifts the wheel into the arch. Driveways at an angle combine lock and compression at once. Check those extremes after any bold fitment change. A light rub mark tells you the story early, before damage grows.
Reading Wheel Stamps and Listings
Wheel markings pack several facts into a short code. A stamp like 8Jx18 ET35 tells the width, diameter and offset together. The letter J describes the rim flange shape used on most road wheels. Online listings sometimes hide offset deep in a specification table. Sellers may quote offset for one variant while picturing another. Always confirm the exact figure for the exact wheel you will receive. A quick message to the seller prevents the most common ordering mistake.
Verdict Bands and What They Mean
The verdict band turns the poke figure into plain advice. Changes inside 10 millimetres either way count as near stock fitment. Increases between 10 and 25 millimetres count as a noticeable poke increase. Beyond 25 millimetres, the stance counts as aggressive and needs fender checks. Tucks between 10 and 25 millimetres pull the wheel further inward. Beyond that, the wheel sits much more inset, and inner clearance becomes the main worry.
Poke, Law and Practicality
Many regions limit how far a tyre may sit beyond the bodywork. Rules differ, yet strongly poked wheels attract attention almost everywhere. Extra poke also throws more road spray along the car’s sides. Mud flaps or arch extensions sometimes solve both problems neatly. Daily drivers usually prize restraint over drama here. Show builds accept the trade offs with open eyes.
Use the bands as a shopping filter before you fall for a design. Wheels that land in the near stock band rarely bring drama. Wheels in the outer bands demand homework, measurements and sometimes arch work. Neither answer is wrong for the right car and owner. The band simply tells you how much homework the choice carries with it.
Insurance and Inspection Notes
Some insurers ask about wheel changes when you modify a car. A fitment far from factory figures counts as a clearer modification. Annual inspections in some regions also check tyre coverage by the bodywork. Keeping your setup inside the verdict bands avoids most of these conversations. Documentation helps too, so save the listings and figures for both wheel sets. Honest records make renewals and inspections boring, which is exactly what you want.
Staggered and Square Setups
A staggered setup runs wider wheels at the rear than at the front. It suits powerful rear wheel drive cars chasing traction and presence. A square setup uses identical wheels everywhere, which simplifies rotation and spares. Each setup needs its own pass through the calculator per axle. Never assume the rear result mirrors the front result. Arches, brakes and factory offsets often differ between the two ends.
Practical Tips Before You Buy Wheels
- Read width and offset from the wheel itself, not from memory or adverts.
- Measure arch and suspension clearance with the suspension loaded, not jacked up.
- Remember that tyre size changes clearance even when the wheel fits.
- Test fit one wheel first when the poke change looks large.
- Avoid spacers as a cure for a badly chosen offset.
- Keep the old figures recorded in case you return to stock wheels.
- Check steering lock both ways after any fitment change.
Frequently Asked Questions
Offset Basics Questions
1. What does ET mean on a wheel?
ET is the German short form used for offset on most wheels. The number after it gives the offset in millimetres. ET35 means a positive offset of 35 millimetres. You will find it stamped or cast into the back of the wheel.
2. Is a higher offset tucked in or poked out?
A higher positive offset tucks the wheel further under the arch. A lower or negative offset pushes it outward. Moving from plus 45 to plus 30 shifts the wheel out by 15 millimetres, before width changes count. The calculator combines both effects for you.
3. Why does the formula add one inch to width?
The stated width measures between the bead seats, not the rim edges. Flanges add roughly half an inch on each side of the wheel. Using width plus one inch gives the true total the car must clear. Fitters have used this convention for backspacing for decades.
4. Can two wheels share an offset but fit differently?
Yes, width changes everything around a fixed offset. A wider wheel grows inward and outward from its centre line. So a 9 inch ET35 wheel pokes more than an 8 inch ET35 wheel. Always compare width and offset as a pair.
5. What offset do most family cars use?
Many family cars sit between plus 35 and plus 55. Exact figures vary by model, trim and brake package. Your current wheel’s stamp is the safest reference. The owner’s handbook or a dealer can confirm the factory range.
Fitment and Clearance Questions
6. What poke change is usually safe?
Changes within about 10 millimetres rarely cause trouble on stock arches. Larger moves can still work with the right tyre and arch room. Every car differs, so treat the verdict bands as guidance. A careful tape measure session beats any general rule.
7. What does a negative inner clearance change warn about?
It warns that the new wheel reaches further toward the suspension. Struts, control arms and brake lines live in that space. Clearance can vanish at full steering lock or full compression. Check the inside as carefully as the arch lip.
8. Do I need new tyres when offset changes?
Not always, since many tyres stretch or pinch within safe limits. A much wider wheel may demand a wider tyre to sit correctly. Tyre shoulder width then changes your real clearance again. Plan wheel and tyre sizes together for the cleanest result.
9. Can spacers fix a wheel that tucks too far?
Spacers reduce the effective offset and push the wheel outward. Quality hub centric spacers with correct bolts can work safely. They also add cost, weight and another part to maintain. Choosing the right offset from the start is cleaner.
10. Will a bigger poke change affect handling?
Moving wheels outward widens the track, which can steady cornering slightly. It also increases load on wheel bearings and steering parts. Scrub radius changes can alter steering feel and kickback. Mild changes rarely bother road cars in normal use.
Buying and Checking Questions
11. Where do I find my current wheel figures?
Look for stamps on the back of a spoke or the hub area. They list width, diameter and ET in one cluster. A tyre shop can read them during a rotation visit. Photograph them for later wheel shopping trips.
12. Should front and rear wheels match?
Many cars run identical wheels at both ends from the factory. Some sports cars use wider rears for grip and looks. This calculator handles one axle at a time. Run it twice whenever front and rear fitments differ.
13. What if the new offset is negative?
Enter it with its minus sign and the calculator handles the rest. Negative offsets push the wheel strongly outward. Backspacing shrinks while outspacing grows by the same logic. Watch the verdict band closely on such setups.
14. Does brake size affect wheel choice?
Yes, large brakes need room behind the spokes and rim. Backspacing alone cannot prove a wheel clears a caliper. Spoke shape matters as much as the offset figure. A physical test fit remains the gold standard for big brake cars.
15. Can I trust the verdict without measuring?
The verdict is a strong screening tool, not a final guarantee. Arch shapes, ride height and tyre choice all vary between cars. Use the calculator to shortlist wheels quickly and safely. Then confirm the winner with measurements or a test fit.
CONCLUSION
Good fitment is a numbers game, and now you hold the numbers. Compare width and offset here to see backspacing, outspacing and the true poke change before money moves. Respect the inner clearance figure as much as the stance you want. Mild changes suit daily cars, while bold ones demand checks and patience. Enter your current and future wheels above and plan the upgrade with confidence.